This invention relates to engine fuel injection systems, and particularly to those of the type in which an electromagnetic fuel injector having an open condition and a closed condition controls the flow of fuel from a constant pressure source.
In one form of such a fuel injection system, apparatus is provided to generate fuel injection pulses having durations determined according to the fuel requirements of the engine. Most commonly the injection pulses are timed synchronously with crankshaft rotation, although asynchronous timing is also known. The apparatus is further effective to apply the fuel injection pulses to the fuel injector to activate it to its open position for the durations of the pulses and to close it between the pulses. Since the flow of fuel through the injector is essentially constant once stable flow is established in the injector's fully open position, the durations of said pulses substantially control the time rate of fuel flow to the engine.
At the beginning and end of each of the fuel injection pulses, however, while the injector is opening and closing and the fuel flow is in a transient condition, the fuel flow rate through the injector varies from the fuel flow per unit time when the injector is fully open. Thus, a portion of the pulse is characterized by a variable flow rate other than the constant rate. When the fuel injection pulses are relatively long in duration, this introduces little inaccuracy in relation to the total fuel flow rate. However, at low engine speeds and loads, when the time rate of fuel flow is small and the fuel injection pulses thus must be comparatively short, the inaccuracy is a greater fraction of total fuel flow.
It is, therefore, useful to define a minimum injection pulse duration, below which an undesirable degree of inaccuracy is introduced into the fuel flow rate. For a particular engine, if such a minimum desired fuel injection pulse duration exists and if the corresponding fuel flow is greater than a fuel flow rate expected in the normal course of operating such an engine, such as during engine overrun or coast down conditions, it is desirable to provide some means to eliminate this inaccuracy or at least minimize its effect on engine operation.